Dynamic changes of microbial communities during natural solid-state fermentation of soybean meal and isolation of dominant bacteria for peptide production

This study aimed to investigate the dynamic changes in bacterial communities during natural solid-state fermentation (NSSF) of soybean meal (SBM) and isolate dominant bacteria for peptide production. Results revealed that during NSSF, bacterial diversity gradually decreased, with Bacillus (particularly Bacillus subtilis) dominating the microbial population (over 90%). Three dominant strains were isolated from the fermented SBM and identified as Bacillus subtilis SBM_1, Bacillus licheniformis, and Bacillus subtilis SBM_2, respectively. Bacillus subtilis (Strains 1 and 3) could secrete extracellular protease, amylase, and cellulase at a relatively high temperature (45 °C). In contrast, Bacillus licheniformis (strain 2) could withstand temperatures up to 55 °C but showed limited protease and amylase activities. Furthermore, among the three dominant strains, strain 1 exhibited the highest protein conversion rate of 34.57% after incubating in casein medium for 24 h, followed by strain 3 (18.97%). The peptide yield of the fermented SBM after inoculating three dominant strains significantly (p < 0.05) increased by 23.47%, 7.27%, and 12.20%, respectively, compared with naturally fermented SBM. As a thermophilic peptide-producing strain, strain 1 (named Bacillus subtilis SBM_1) possesses the potential for improving the added value of the SBM in the SSF industry.